Expansible mandrels for use in driving and withdrawing tubular piles have lengthwise sections the upper ends of which are interconnected by a head. Means of one type or another are employed to force the sections apart, after the mandrel has been bottomed in a pile, into such contact with the interior of the pile that the pile is locked to the mandrel means of one type or another are also employed to effect the re-establishment of the pile-entering relationship of the sections once the pile has been driven to the wanted depth.
The most effective means for effecting the pile-gripping relationship of the sections utilizes a steel cable or cables reeved over groups of sheaves which bear alternatively on the interior of the two mandrel sections. The cable courses are so reeved relative to the sheaves of each group and so connected to the mandrel and to hydraulically operated means that when the latter is operated exert a straightening pull on the courses in the sheaves and accordingly the mandrel sections are moved apart with great force into their pile-gripping relationship. The present invention is illustrated with means of this type as such exert forces over substantially the full length of the mandrel ensuring that the mandrel and the pile are so securely coupled as to effect successful pile driving even with unfavorable soil conditions.
To effect the pile-entering relationship of the mandrel members, the members have been interconnected in a plurality of zones spaced lengthwise of the mandrel by resilient means either utilizing compression springs or leaf springs.
Resilient means of the compression spring type required that in each zone, the mandrel sections be interconnected at each side of the channel for the cable or cables by a rod extending through oppositely opening, aligned sockets with nuts threaded thereon by which the compression springs are held bottomed in their sockets. This construction, while satisfactory at the boot end of a mandrel and capable of retracting the mandrel sections if a sufficient number of such interconnections are employed, has the objectionable feature that bores were required through the sections which result in troublesome stress rises.
Resilient means of the leaf spring type, see U.S. Pat. No. 3,799,026, required that each leaf spring be located centrally of the mandrel sections with the ends of each spring connected to mounts spaced lengthwise of one section with the cable courses bridged by the mounts. The other mandrel section has a member underlying the central portion of each spring causing the spring to become tensioned as the mandrel sections are forced apart into their pile-gripping relationship.
While leaf springs have the advantage that the retracting forces of each is applied over a substantial length, they have the disadvantages that they are not adjustable and are relaxed except when expansion forces are exerted.
Another system of re-establishing the pile-entering relationship of the sections was to employ a cable-sheave arrangement such with a pull on the retraction cable as by hydraulically operated means, the mandrel sections were positively forced together. Such a system while effective to ensure the freeing of the mandrel sections from the driven pile added substantially to the cost of the mandrel and increased assembly and servicing problems.